My lemon tree won’t grow lemons

I purchased a lemon tree from the Tampa Airport gift shop back in 2008. It was one stalk with a few leaves. Now it has grown into a beautiful lemon "bush". It is approx. 1 foot high but very full like a bush. It has never produced any lemons. The leaves smell very fragrant with a lemon scent and the bush has a lot of thorns.

I live in Michigan and it is brought indoors for the winter months. I also have used citrus dirt when replanting my lemon tree to a larger container. I also have read to add coffee grounds occasoinally to help, but so far no luck. No blossoms and no lemons. I am not sure what brand of lemon tree this is, but was hoping for any advice you can give to help me produce lemons. Thanks!